Validating environmental flow recommendations: drifting coarse particulate matter, macroinvertebrates, and larval fishes
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Water quantity management in Texas and elsewhere is currently managed under the theory of the Natural Flow Paradigm, which states that sound ecological riverine environments are dependent upon the dynamic character of flow. Water quantity recommendations and standards prescribe a multi-tier flow regime, consisting of subsistence, base, and high-flow pulses, with magnitudes of each based on long-term averages of site-specific hydrographs. The next step in water quantity management is to validate that the recommended flow regimes are sufficient to maintain a sound ecological environment, although validation methodologies are rarely incorporated into water quantity management plans. Purposes of this study were to develop methodologies for validating flow recommendation and standards that are transferable and replicable and to quantify the value of flow tiers relating to organic drift. Objectives of this study were to assess drift biomass of coarse particulate matter (CPM) and drift rates of macroinvertebrates and larval fishes related to flow tiers (i.e., subsistence, base, two per season high flow pulse, one per season high flow pulse, and one per year high flow pulse) across four sites and two rivers (i.e., San Antonio and Guadalupe rivers). A priori predictions were that CPM, macroinvertebrates, and larval fishes were in greater biomass or densities at higher levels of flow tiers, though at some point flows would be sufficiently high and induce a washout effect. Findings were generally inconsistent with the predications. Biomass of CPM did not differ among flows ranging from subsisted to two per season high flow pulse events, and densities of macroinvertebrate did not differ among flows ranging between base and one per season events. However, densities of fluvial specialist Neoperla (Order Plecoptera) were positively correlated with flow as a continuous variable rather than a categorical variable. Densities of larval fishes were greatest at subsistence, base, and two per season high flow pulses and decreased at 1 per season flow pulses. It is uncertain if a washout effect occurred or if survival of larval fishes was decreased at a higher flow pulses. With a validation methodology in place, replications from other sites and river basins can be added to the existing model to improve statistical power and inference, along with other flow-dependent variables, in order to fully assess the value of multi-tier flow regimes.